Support ssh-ed25519 SSH keys
Summary: Fixes T7121.
Test Plan: Used ssh-keygen -t ed25519 on an Ubuntu 14 box to generate a key; verified this is the header on the corresponding public key.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7121
Differential Revision: https://secure.phabricator.com/D11930